The Volcano Acoustic Source was motivated by the observations that volcano sounds may change in character leading up to violent eruptions.  In particular this project focuses on the study of infrasound, which are low-frequency sound waves produced by volcanoes that are inaudible to humans, but are especially intense.  These infrasounds are also able to radiate to distant monitoring sites with very little signal decay.  This permits remote, convenient and continuous analysis of the conditions within a volcanic crater even when it may be obscured by cloud or nighttime.

            Marked changes in infrasound frequency and resonance quality (timbre) preceded the March 3, 2015 eruption of Volcan Villarrica (in Chile) when the lava lake surface ascended within the summit crater over the course of two days.  Monitoring stations more than 4 km from the summit were used to detect the changes in infrasound character and to develop a preliminary model for how infrasound may be used to forecast eruptions at some open-vent volcanoes.  The premise of the Volcano Acoustic Source: Decoupling Crater Modulation is that crater shape, including its depth, aspect ratio, and symmetry, presents an important control on the (infa)sound that is projected from a crater.  A crater may be considered to be analogous to a giant musical horn that emits resonant sounds, but of very low frequency and very long wavelengths.  When the shape of the crater changes the character of the infrasound is affected.

            The primary scientific discoveries from the Volcano Acoustic Source is an understanding of the influence of crater shape on volcanic sound radiation.  Through a combination of computer-based numerical modeling and field data collection, including drone-based mapping, we were able to accurately measure volcanic craters and simulate the sounds that they can produce.  A study utilizing field data collected at Villarrica in 2020 was able to explain the characteristic frequency, or 'voiceprint' of that volcano [Rosenblatt et al., 2022].

            Another study improved upon the motivating observations of changing infrasound frequency leading up to a paroxysm, which is a particularly violent eruption.  This case study from Etna (Italy) [Sciotto et al., 2023] observed 16 hours of systematic changes in frequency, prior to a significant lava fountaining erupting to heights of more than a kilometer.  Combined modeling and understanding of the crater acoustic response permitted an estimation of the changing lava column's location within the volcanic crater prior to the lava fountain.

            A final consequential study was somewhat serendipitous in its discovery.  During the field work to map, model, and record the infrasound from Yasur Volcano (Vanuatu) coincident camera observations were used to visually extract infrasound from volcanic plume vaporization and condensation [Johnson et al., 2023].  The discovery of remarkable capabilities of conventional cameras to record propagating infrasound from explosions and from resonance gives promise for implementation of a novel volcano opto-acoustics tool capable of spatial sampling of the infrasound wavefield.

            This project involved myriad broader impacts such as training of U.S.-based students and other early career collaborators, including international partners from Chile, Italy, Vanuatu, and New Zealand.  U.S. graduate students were given the opportunity to engage in international volcano acoustics field work, instrument preparation and development, and digital signal processing and analysis.  The project helped facilitate the development and implementation of low-cost infrasound sensing technologies, including large-N deployments of low-cost sensors developed at Boise State University.  This in turn has contributed to the proliferation of infrasound sensing technology in other geoscience disciplines.  Our project also succeeded in development of a real-time infrasound-to-audio-band conversion device such that humans can directly perceive volcanic infrasound in the field.

            A short outreach video showing students in action and brief results of volcano infrasound geophysics may be found at http://volcanosciencestories.info.  Additional outreach materials can be found at https://sites.google.com/view/jeffreybjohnson/infravolc

References cited include:

Johnson, J. B., Boyer, T., Watson, L. M., & Anderson, J. F. (2023). Volcano Opto‐Acoustics: Mapping the Infrasound Wavefield at Yasur Volcano (Vanuatu). Geophysical Research Letters, 50(8), 1?10. https://doi.org/10.1029/2022GL102029

Rosenblatt, B. B., Johnson, J. B., Anderson, J. F., Kim, K., & Gauvain, S. J. (2022). Controls on the frequency content of near-source infrasound at open-vent volcanoes: a case study from Volc?n Villarrica, Chile. Bulletin of Volcanology, 84(12), 103. https://doi.org/10.1007/s00445-022-01607-y

Sciotto, M., Watson, L. M., Cannata, A., Cantarero, M., De Beni, E., & Johnson, J. B. (2022). Infrasonic gliding reflects a rising magma column at Mount Etna (Italy). Scientific Reports, 12(1), 16954. https://doi.org/10.1038/s41598-022-20258-9

Last Modified: 06/25/2023
Modified by: Jeffrey B Johnson